In this episode Shahriar repairs an Agilent PSA Series Spectrum Analyzer. The instrument generates many errors during self-alignment and produces no measurements below 3.2GHz. The block diagram of the unit is thoroughly presented and various possible failure points are considered. Based on the observation of the noise floor, the most likely cause is the second LO module. The measurement of the LO power indicates that the second LO power is fall below nominal.
The second LO signal (at 3.6GHz) is generated by a DRO PLL module which is locked to 600MHz. The PLL is functional, however the output power is below -20dBm. Teardown of the module reveals a simple design with a doubler and filter. The filter (which is discolored) shows a huge loss likely caused by RF losses due to trace degradation. Thinning the traces provides some improvement and the remaining loss is compensated with a two stage RF amplifier. With this modifications, the instrument’s alignment errors are partially resolved.
The next problem is with the input attenuator at the 20dB range. This problem is simply resolved by providing lubrication on the attenuator solenoids after disassembly. The instrument is then used for various measurements to verify its correct operation.
In this episode Shahriar investigates the noise generated from the DPH3205 Buck-Boost DC/DC Converter Module. This module is part of a family of affordable devices which can be purchased online. The particular model (DPH3205) shown in this video can be purchased from here. The Keysight S-Scope in conjunction with the N7020 power probe is used to measure the noise behavior of the circuit. It can be observed that the module produces many harmonics of noise spanning up to and beyond 1MHz.
A power operational amplifier is used in conjunction with a 4V Zener diode to produce a crude voltage follower. The PSRR of the operational amplifier can filter the noise from the DC/DC converter. This is verified by measuring the noise coming from the operational amplifier. The noise is significantly reduced. This circuit is not entirely practical and is used for demonstration purposes only.
In this episode Shahriar repairs an Anritsu MS9710B Optical Spectrum Analyzer. The instrument intermittently does not boot and when it does it generates a Grating Mirror error. Some investigation reveals that the problems may simply be poor internal cabling connections which can be resolved by cleaning all connectors and re-building the unit. The LCD screen’s CCFL have also failed. An LED back-light replacement provides the screen with a vibrant and bright colors.
The unit is very compact (which is unusual for an Optical Spectrum Analyzer) and makes it ideal for bench-top working environments. The broad-spectrum built-in reference laser can be used to calibrate and align the instrument which proves to work perfectly. After calibration, a laser diode from an Agilent Lightwave Transmitter is measured. Interestingly it can be seen that the unit produces two tones at ~1nm apart which indicates the laser operates in two modes. A very interesting result given that the wavelength meter only reports one tone at the average wavelength of the two modes.
In this episode Shahriar investigates a peculiar problem with an Agilent P-Series Power Meter. While the instruments works during startup, after about 10 minutes the LCD screen begins to flicker and become scrambled. Initial investigation reveals that the issue is not likely with the main motherboard since USB connection to the instrument and data-readout is possible even when the LCD screen is malfunctioning.
Teardown of the unit reveals a modular design where a ribbon cable connects the front panel to the motherboard. Moving the cable around affect the LCD which points to a faulty cable connection. The failure is at the only ribbon cable without a proper removable connector to the motherboard. The cable is cut, re-crimped and the solder joints re-worked. This appears to solve the LCD problem. The performance of the unit is verified by measuring the output power of the Tektronix TSG Vector Signal Generator.
In this episode Shahriar explores the cause of failure of an Agilent E4405B ESA 13.5GHz spectrum analyzer. In a previous episode, an E4407B version of the ESA was repaired and upgraded which is helpful in debugging and analyzing this instrument. The instrument displays LO Unlock which is a familiar problem. After an overview of the system block diagram, the LO signal path is traced and analyzed for problems.
After some investigation, the problem appears to be identical to the E4407B model repaired before! The Hittite (Analog Devices) divide-by-4 static divider unit has failed and does not divide correctly at high frequencies. The component is replaced which restores the instrument’s functionality.
The LCD screen of the unit is also very dim. The CCFL tube and the inverter are replaced with an LED back light instead. The unit then produces a bright and vibrant graphs and text. The performance of the instrument is verified with a Keysight MXG signal generator.
The Signal Path (TSP) is an electrical engineering video blog for industry professionals, students and hobbyists. TSP is a non-for-profit website dedicated to provide free education spanning a wide range of electrical engineering topics. Equipment reviews, tutorials and repair videos are posted regularly.